Audio systems that are powered by a USB bus supply require a tradeoff between the peak output power delivered to the speaker loads and proper compliance with USB standards. The USB standard requires the USB host to supply a minimum of 500 mA of continuous current, with brief support of higher peak levels of limited duration. It is also the USB host's responsibility to detect when a device connected to any individual port violates the current limit and to act appropriately to protect the host as well as other devices connected to other USB ports. The offending port can be reset to see if that corrects the excessive current draw. If the current limit is repeatedly violated, the port could be disabled and the user would be notified that something is wrong and that the port will have to be manually reactivated.
In contrast to these host limitations, marketing directives push for the “loudest” or highest power audio system feasible (where power is proportional to current). Since the USB restrictions are rigid requirements, it is necessary to maximize output power without violating the USB current limitation. However, prior art solutions that violate the current limit often result in a device reset, which causes the audio device to become disconnected from the USB host and requires the device to be re-initiated by the host. When the device resets, the audio stream ends abruptly and the user has to restart the playback. Alternatively, if the device doesn't reset, the USB host can reset the port itself, resulting in similar consequences. If multiple resets occur, the USB host could ultimately disable the port completely.
In order to avoid the issues described so far, two basic solutions have been seen: either limit the output level at all times to avoid ever drawing too much current, or add current limiting between the host and amplifier. The former disproportionately trades performance for compliance, while the latter adds considerable cost to the system. An alternative approach that is commonly seen it to add current detection in the amplifier's output to detect excessive load current draw. While this approach can handle gross situations like short-circuits, it is difficult to optimize load power because there usually isn't a simple relationship between the load current and supply current. Intermediate thresholds can be achieved, but the issue is that the load current isn't a precise measure of the supply current. Because the supply current is related to the load current via efficiency calculations, and the efficiency isn't well known in every condition, this method requires the designer to use the most conservative estimate of efficiency, thus resulting in a less than optimal design.